Arc discharge lamps, and in particular metal halide lamps, are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes an arc discharge vessel made from quartz or fused silica or a ceramic such as polycrystalline alumina that is hermetically sealed within a borosilicate glass outer envelope. The arc discharge vessel, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps and lamps with ceramic arc discharge vessels, the outer envelope is evacuated.
The arc in a standard metal halide lamp that is operated horizontally bows upward due to convection, thus the arc is not centered within the arc discharge vessel but is raised toward the upper wall. This condition raises the temperature of the arc discharge vessel material that is nearest to the arc. The overheated upper arc discharge vessel wall results in reduced lamp life. Additionally, a bowed arc makes it difficult to focus in applications where optics are important.
It has been proposed to alleviate this problem by bowing the arc discharge vessel to approximate the shape that the arc will take. This latter technique is exemplified in U.S. Pat. No. 3,858,078. While this solution works well and has been in use for many years, the lamp employing a curved arc discharge vessel is limited to horizontal operation and must have a special socket with a particular orientation to insure that the bowed portion of the arc discharge vessel is upward when installation is complete. Additionally, it has been found to be difficult if not impossible to utilize the bowed arc discharge vessel design in two-piece, injection molded ceramic arc discharge vessels because the curved shape would not permit removal from the mold, This is unfortunate because the ceramic arc discharge vessel has many advantages over the quartz type and it would be desirable to use such arc discharge vessels in horizontally operated lamps.
Another technique for straightening the arc employs specialized, acoustic ballasts; however, these ballasts are more complicated than and more expensive than standard metal halide ballasts. A further problem with these acoustic ballasts is that they can have a detrimental effect on the stability of the lamp operation. These ballasts are often used with mercury-free metal halide lamps; however, removing mercury causes a voltage drop that makes it is necessary to increase the arc length. Without the acoustics however, the mercury-free arcs are less stable than the shorter, mercury-containing arcs.